U.S. patent number 7,622,909 [Application Number 10/545,984] was granted by the patent office on 2009-11-24 for magnetic field sensor and electrical current sensor therewith.
This patent grant is currently assigned to LEM Heme Limited. Invention is credited to Wolfram Teppan.
United States Patent |
7,622,909 |
Teppan |
November 24, 2009 |
Magnetic field sensor and electrical current sensor therewith
Abstract
A magnetic field sensor comprises a magnetic field sensing cell
and a magnetic shield comprising at least two parts separated by an
air-gap and surrounding the magnetic field sensing cell positioned
in a cavity of the magnetic shield.
Inventors: |
Teppan; Wolfram
(Collonges-s-Saleve, FR) |
Assignee: |
LEM Heme Limited (Lancashire,
GB)
|
Family
ID: |
32731555 |
Appl.
No.: |
10/545,984 |
Filed: |
February 18, 2004 |
PCT
Filed: |
February 18, 2004 |
PCT No.: |
PCT/IB2004/000580 |
371(c)(1),(2),(4) Date: |
August 18, 2005 |
PCT
Pub. No.: |
WO2004/074860 |
PCT
Pub. Date: |
September 02, 2004 |
Prior Publication Data
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|
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Document
Identifier |
Publication Date |
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US 20060226826 A1 |
Oct 12, 2006 |
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Foreign Application Priority Data
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|
|
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Feb 21, 2003 [EP] |
|
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03003879 |
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Current U.S.
Class: |
324/126;
324/117H |
Current CPC
Class: |
G01R
33/04 (20130101); G01R 15/207 (20130101); G01R
15/202 (20130101); G01R 15/185 (20130101) |
Current International
Class: |
G01R
1/20 (20060101); G01R 15/20 (20060101); G01R
33/07 (20060101) |
Field of
Search: |
;324/117H,117R,126,127 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Nguyen; Ha Tran T
Assistant Examiner: Velez; Roberto
Attorney, Agent or Firm: Browning; Clifford W. Krieg DeVault
LLP
Claims
The invention claimed is:
1. Electrical current sensor for measuring the electrical current
I.sub.p flowing in a primary conductor (1), comprising a magnetic
circuit (3) having an air-gap (6) and a magnetic field sensor
comprising a magnetic field sensing cell (10, 310) and a magnetic
shield (9, 109, 209, 409) made of a magnetic material comprising at
least two parts (12, 112, 212, 412) separated by an air-gap (20)
and surrounding the magnetic field sensing cell positioned in a
cavity (11) of the magnetic shield positioned in the air-gap,
wherein the magnetic circuit comprises a core (5) of soft magnetic
material surrounding the primary conductor, end faces (27a, 27b) of
the core having extensions (28a, 28b) that reduce the width of the
air-gap along an outer periphery of the magnetic core.
2. Electrical current sensor for measuring the electrical current
I.sub.p flowing in a primary conductor (1), comprising a magnetic
circuit (3) having an air-gap (6) and a magnetic field sensor
positioned in the air-gap, the magnetic field sensor comprising a
magnetic field sensing cell (10, 310) and a magnetic shield (9,
109, 209, 409) made of a magnetic material comprising at least two
parts (12, 112, 212, 412) separated by an air-gap (20) and
surrounding the magnetic field sensing cell positioned in a cavity
(11) of the magnetic shield the current sensor further comprising a
circuit board (22) with conductive tracks thereon wherein the
magnetic circuit (3) and the magnetic field sensor (4, 104, 204,
304, 404) is fixed directly on the circuit board, and wherein the
magnetic field sensor comprises a flexible circuit (14, 214, 414)
that is conductively connected to conductive traces on the circuit
board that connect the magnetic field sensor to an ASIC (23)
mounted on the circuit board for processing the sensor input and
output signals.
Description
Applicant claims foreign priority benefits under 35 U.S.C.
.sctn..sctn. 119(a)-(d) or (f), or .sctn.365(b) of European Patent
Application No. 03003879.8, filed Feb. 21, 2003.
This invention relates to a magnetic field sensor for measuring
magnetic induction (magnetic flux density). This invention also
relates to an electrical current sensor for measuring the
electrical current flowing in a conductor by sensing the magnetic
field generated by the conductor.
Many conventional current sensors have a magnetic circuit
comprising a core of magnetic material provided with an air-gap in
which a magnetic field sensor, such as a Hall effect sensor, is
positioned. The conductor in which the current to be measured
flows, often called a "primary conductor", passes through the
magnetic circuit one or several times. Such sensors are commonly
found in applications where relatively large electrical currents
are measured, or in applications where simple devices, such as
current shunts or transformers, cannot be used for reasons such as
the need for galvanic isolation or the presence of D.C. components
in the current to be measured.
Current sensors of the aforementioned type can usually be
categorized into three groups: closed-loop sensors, open-loop
sensors, and a group of modified open-loop sensors that use a
transformer effect for high frequencies.
Closed-loop sensors rely on the principle of compensation of the
current linkage generated by the primary conductor through which
the current to be measured flows by means of a secondary coil that
is driven by an electrical circuit controlled by the magnetic field
sensor placed in the magnetic circuit air-gap. As a result, the
magnetic induction in the air-gap of the magnetic circuit is
adjusted to be as small as possible in order to obtain a negligible
measurement error.
Open-loop sensors rely on the measurement of magnetic induction in
the air-gap of the magnetic circuit surrounding the primary
conductor. The magnetic induction in the air-gap is proportional to
the current to be measured as long as the magnetic permeability of
the magnetic circuit is much higher than that of air or other
materials filled in the air-gap that have permeability values close
to that of air (which is approximately equal to 1). In modified
open-loop sensors with transformer effect, low frequency currents
are measured in the same way as the above described open-loop
sensors, whereas high frequencies are measured by a secondary
winding connected to a current measuring shunt. High frequency
components of the primary current are transferred to the secondary
winding by a transformer effect, whereby the signal of the shunt is
added electronically to the signal of a device measuring the
induction air-gap. This enables the usable frequency range of
open-loop sensors to be extended, since unmodified open-loop
sensors are typically unable to follow the steeply-rising currents
of high frequencies because of Eddy current effects in the magnetic
core and the limited bandwidth of common magnetic field
sensors.
Open-loop current sensors are generally less complicated and less
costly than closed-loop sensors, however they have the disadvantage
of being less linear and stable and thus have a higher measurement
error.
Magnetic field sensors, such as Hall effect sensors, exhibit high
offset and gain drifts with respect to temperature, with the
further disadvantage of being sensitive to mechanical stress.
Such problems limit the use of Hall effect sensors in applications
with relatively high temperatures, or subject to mechanical
stresses, for example due to relative thermal expansion of
components on which the sensor is mounted. Such problems occur for
example in Integrated Power Modules (IPM) that have semi-conductor
power chips mounted on a ceramic substrate, often called "DCB"
(Direct Copper Bonding) technology. During operation of IPM's,
temperatures in a range of 120.degree. C. are common, such
temperatures being problematic for stable and linear operation of
Hall cells without offset drift.
Other known magnetic field sensors, such as fluxgate sensors or
magneto-impedance sensors, are more stable and have low offset and
gain drift compared to Hall sensors. They are however limited by
their magnetic induction measuring range which is in the order of
several milli-Tesla (mT), whereas in the air-gap of open-loop
current sensors, induction levels of more than 100 mT are
common.
There are many applications, in addition to those related to
electrical current measurement, where it is desirable to measure
magnetic inductions in adverse temperature and/or mechanical stress
situations.
Considering the aforementioned disadvantages, an object of this
invention is to provide a current sensor, that has a high degree of
linearity and stability over a large operating range, and that is
subject to low offset and gain drift. It is further advantageous
that such sensor be cost effective and compact.
Another object of this invention is to provide a magnetic field
sensor that has a low offset and gain drift and that is able to
measure large magnetic induction levels. It is further advantageous
to provide a magnetic field sensor that is robust and that can
operate accurately over a large range of temperatures.
Objects and advantageous features of the invention will be apparent
from the following description, claims and drawings, in which:
FIG. 1 is an exploded view in perspective of a first embodiment of
a magnetic field sensor according to the invention;
FIG. 2a is a partial cross-sectional view in perspective of a
sensing cell of a magnetic field sensor according to the
invention;
FIG. 2b is a view in perspective of an underside of the sensing
cell shown in FIG. 2a;
FIG. 3a, 3b and 3c, are views in perspective of variants of soft
magnetic cores of sensing cells according to this invention;
FIG. 4 is a view in perspective of an embodiment of a current
sensor according to this invention;
FIG. 5 is a plan view of an embodiment of a magnetic circuit of a
current sensor according to this invention, with a magnetic shield
positioned in an air-gap of the magnetic circuit, showing magnetic
flux lines B;
FIG. 6 is a detailed cross-sectional view of the magnetic shield of
the embodiment of FIG. 5;
FIG. 7 is a view in perspective of an embodiment of a packaged
electrical current sensor according to this invention;
FIG. 8 is an exploded view in perspective of a second embodiment of
a magnetic field sensor according to the invention;
FIG. 9 is an exploded view in perspective of a third embodiment of
a magnetic field sensor according to the invention;
FIG. 10 is a partial, cross-sectional view of a magnetic field
sensor according to another embodiment of the invention;
FIG. 11 is a view in a direction of arrow X of the magnetic field
sensor of FIG. 10 with the upper half of the magnetic shield
removed; and
FIG. 12 is an exploded view in perspective of a packaged electrical
current sensor according to this invention.
Referring to the figures, an electrical current sensor 2 for
measuring a primary current I.sub.p in a primary conductor 1,
comprises a magnetic circuit 3 and a magnetic field sensor 4, 104,
204, 304, 404. The magnetic circuit 3 comprises a magnetic core of
a magnetically permeable material and is provided with an air-gap 6
between end faces 27a, 27b in which the magnetic field sensor is
positioned. The magnetic core 5 is shown as a generally
annular-shaped part with a central opening 7, through which the
primary conductor 1 extends. Other magnetic circuit shapes could
however be provided, such as rectangular, polygonal, toroidal or
otherwise, without departing from the scope of this invention.
Moreover, the primary conductor 1 is shown as extending through the
magnetic circuit, however, the primary conductor could also be
provided with a number of windings or conductor portions wrapped
around the magnetic circuit core 5 and extending through the
opening 7. The magnetic material of the magnetic core 5 may be
provided with the properties found in known current sensors of this
general type and may have a laminated construction to reduce eddy
currents.
As best seen in FIG. 4 or 5, the end faces 27a, 27b of the magnetic
core 5 advantageously each have an extension 28a, 28b at the outer
periphery of the core that reduces the width of the air-gap. These
outer extensions advantageously reduce the effect of interference
by magnetic fields generated outside the current sensor by
conducting the external magnetic flux along the outer periphery of
the magnetic core, thus avoiding the magnetic field sensor 4, 104,
204, 304, 404. For reasons of symmetry, extensions 29a, 29b may
also be provided along the inner periphery of the magnetic core
5.
The current sensor may have the general characteristics of an
open-loop current sensor, in that there is no secondary coil or
conductor for compensating the ampere turns generated by the
primary conductor as found in closed-loop sensors. The sensor may
however be provided with a secondary winding 8 around the magnetic
core 5 for detecting high frequency currents by the transformer
effect as shown in FIG. 4.
The magnetic field sensor 4, 104, 204, 304, 404 comprises a
magnetic shield 9, 109, 209, 409 made of a soft magnetic material
and a sensing cell 10, 310 positioned in a cavity 11 of the
magnetic shield. The magnetic shield comprises at least two shell
portions 12, 112, 212 separated by a small air-gap 20 therebetween.
The magnetic shield 9 has a generally axisymmetric or cylindrical
shape in the embodiment shown in FIG. 1 and is arranged such that
the central axis of the cylinder is substantially in line with the
magnetic flux lines extending across the current sensor air-gap 6.
The magnetic shield may however be provided with non-axisymmetric
shapes, for example having rectangular (see FIGS. 8, 9 and 12),
polygonal or elliptical cross-sections. It is also possible to
provide the magnetic shield with a non-prismatic shape, for example
as seen in the longitudinal cross section the embodiment of the
magnetic shield 9 shown in FIG. 6.
In the embodiments of FIGS. 8 and 9, the magnetic shield 109, 209
is wholly or partially stamped and formed from sheet metal that may
be annealed after forming to ensure homogeneous and optimal
magnetic properties like high magnetic permeability and low
coercive force. The stamping and forming of the magnetic shield in
high volumes is particularly cost effective compared to machined
shields.
The shell portions 412 of the magnetic shield may also be machined
from a magnetically permeable material, as shown in FIG. 12.
The air-gap 20 may be filled with a material having a low
permeability (in other words a non-magnetic material). Preferably,
the air-gap comprises a material, such as solder or adhesive, for
bonding the pair of shell parts 12, 112, 212, 412 together. A
longitudinal slot 13, 113, 213, 413 extending essentially parallel
to the axis of the magnetic field sensor is provided in the
magnetic shield 9, 109, 209, 309, 409 and serves to reduce Eddy
currents in the soft magnetic shield. The slot may also serve as a
passage into the magnetic shield for a conducting member 14, 214,
414 for connection to the sensing cell 10, 310. In the embodiments
of FIG. 8, the slot 213 is provided by the separation of the
magnetic shield shell parts into first and second portions 212a and
212b that are assembled on opposing sides of a substrate of the
conducting member 214. The thickness of the substrate thus defines
the width of the longitudinal slot 213.
In the embodiment of FIG. 12, the longitudinal slot 413 is machined
in the shell parts 412 and the substrate 414 is inserted into the
slot during assembly.
The conducting member 14, 414 may be in the form of a flexible
printed circuit with conductive tracks 15 for connecting an
electronic circuit 22 (see FIGS. 7 and 12) for processing the
sensor signals to the magnetic field sensing cell 10, 310. As shown
in FIG. 9, the printed circuit may also be provided with metallized
portions 15a for soldering the portions 212a and 212b of the
magnetic shield parts thereto. The portions of the magnetic shield
parts could however also be bonded to the conducting member with an
adhesive or fixed by mechanical means.
The magnetic field sensing cell 10, 310 comprises a coil 16 and a
saturable soft magnetic core 17, 317. The coil 16 is mounted on a
bobbin 18 provided with a groove 19 or cavity 319 in which the soft
magnetic core 17, 317 is mounted. In the embodiments shown, the
coil 16 is made of an insulated electrical wire electrically
connected, via its extremities 16a, 16b bonded to connection pads
16c, to respective conductive tracks 15 of the conducting member
14, 214. The coil and bobbin may be partially overmoulded with a
casing 30, that serves not only to protect the coil, but also to
position the sensing cell within the cavity of the magnetic
shield.
The soft magnetic core 17, 117, 217 is advantageously made of a
fine ribbon of a soft-magnetic material with a thickness preferably
below 0.1 mm. The ribbon 17, 117, 217 may have a cross section that
varies over its length, as shown in the different variants of FIGS.
3a to 3c, in order to achieve magnetic induction values that are
essentially constant all over its volume, which aids to saturate
the ribbon in a very abrupt manner and leads to a current threshold
for the saturation of the fluxgate that can easily be detected by
the associated electronic circuit.
The soft magnetic core 317, as shown in the embodiment of FIGS. 10
and 11, could be made also of a plurality of very fine wires made
of soft magnetic material, bundled or bonded together to form a
generally cylindrical shape, positioned in a cavity 319 of the
bobbin 318. Preferably, the wires are very thin, for example having
diameters in the range of 10 to 50 microns, for example 30 microns,
and bonded together with a dielectric material in order to minimize
Eddy currents. This enables the sensing cell 310 to operate at very
high frequencies without loss of linearity.
The magnetic field sensing cell 10, 310 described above is operated
in a similar manner to known fluxgate sensors. There are different
known techniques by which fluxgate sensors may be controlled by an
electronic circuit to determine the value of the induction to be
measured, such techniques also being applicable to control the
magnetic field sensing cell according to this invention.
Fluxgate sensors are very stable and precise, and have a low offset
drift and gain. The main disadvantage of fluxgate sensors is
however that they have a low magnetic induction operating range,
usually in the order of only a few milli-Teslas, and are therefore
not found in electrical current sensors with large magnetic
fields.
In the present invention, this problem is overcome by placing the
magnetic field sensing cell 10, 310 in the magnetic shield 9, 109,
209 with air-gap 20, that acts as an induction divider. The
magnetic shield 9, 109, 209 conducts most of the magnetic flux, the
proportion depending on the width of the air-gap 20 between the
shell parts 12, 112, 212, 412. The magnetic induction inside the
shield is directly proportional to the magnetic induction present
in the soft magnetic material of the shield, however of a
significantly lower value.
By way of example, if the magnetic shield has a magnetic
permeability of .mu..sub.r=10.sup.5 and the air-gap between shell
parts 12, 112, 212, 412 is 10 microns wide, and the magnetic
induction in the air-gap 6 of the current sensor is 60 mT, the
magnetic induction inside the core 17, 317 of the sensing cell 10,
310 is less than 2 mT.
Advantageously therefore, the magnetic field sensor 4, 104, 204,
304 can be used to measure magnetic inductions as large as 100 to
200 mT, while benefiting from the precision and stability, and the
absence of offset and gain drift, of a fluxgate sensing cell.
Instead of a fluxgate sensor, it is possible to employ other types
of sensitive magnetic field sensing cells that also exhibit low
temperature offset and gain drift. Examples of other such magnetic
field sensing cells that may be used in the present invention, are
so called giant magnetoimpedance sensors or magnetoresistive
sensors of any kind.
It may be noted that the magnetic field sensor according to this
invention may be employed not only to measure magnetic induction in
an air-gap of a current sensor, but also to measure magnetic
induction in other applications, such as magnetic induction
generated in transducers, for example position sensors, proximity
switches or encoders, and other magnetic field systems.
Referring to FIG. 7, an example of a current sensor according to
this invention for use in an integrated power module (IPM) is
shown. The current sensor is shown mounted on a substrate of a
circuit board 22, which may for example be of a ceramic material
with conductive tracks thereon. The magnetic circuit 3 is fixed
directly on the circuit board 22 and surrounds a primary conductor
bar 1 that extends therethrough. The magnetic field sensor, which
may have the construction of any of the aforementioned embodiments,
may also be bonded directly on the printed circuit board, whereby
the flexible circuit 14 thereof is conductively connected to
conductive traces that connect the magnetic field sensor to an ASIC
23 for processing the sensor input and output signals. External
connections to the circuit board may be provided via a connector 24
mounted on the circuit board and pluggable to a complementary cable
connector 25.
Conductors of the secondary winding 8 are, in this embodiment, in
the form of U-shape wires 26 positioned over portions of the
magnetic core 5 and bonded onto circuit traces of the circuit board
22. The secondary coil circuit traces pass underneath the magnetic
core 5 so as to form, with the U-shape wires, essentially a coil
surrounding the magnetic core.
Advantageously, a particularly compact and resistant current
sensor, in particular to high operating temperatures, is
provided.
* * * * *